Geology Reference
In-Depth Information
Fig. 18.22
Schematic illustration of the fl uvial to tidal transition zone showing zones of variable infl uence of the tides (after
Martinius and Gowland
2010
)
and de Boer
1991
; Fig .
18.23
), highlighting the segre-
gation of fl ood- and ebb-current generated structures
around the meander bend.
Tidally-infl uenced deposits of the Castigaleu Fm
are formed in meandering channels and mouth bars
associated with distributary channels (Cuevas Gozalo
and de Boer
1991
, their stop 4). These are typifi ed by
tabular and trough cross-stratifi ed well-sorted sandstone
beds with common reactivation surfaces and occasional
herringbone structures; ripple-laminated sets occur in
the top and mud drapes occur particularly in mouth
bars entering brackish bays (Nijman and Nio
1975
;
Marzo et al.
1988
; Hoornweg
1988
; Fig.
18.21e, f
).
Some examples contain abundant brack-water to
normal marine ichnofacies and oyster beds, and the
top is commonly mottled. The thick fi ne-grained inter-
vals between the sandstone units are deposited in
inshore brack-water lagoons or shallow-water embay-
ments based on body fossil content.
Some isolated meandering channels of the delta
plain of the Montllobat Fm in the Noguera Ribagorçana
River valley, originally described by Puigdefàbregas
and van Vliet (
1978
) and Van der Meulen (
1982
) , as
well as the Gargalluda sandstone complex, a 2 km
wide trunk river alluvial valley stratigraphically 30 m
higher (Marzo et al.
1988
) , show features interpreted
as tidal infl uence on fl uvial accretionary bedforms dur-
ing bankfull stage and reversal after fl ood combined
with low current velocities in the channel (Cuevas
Gozalo and de Boer
1991
, their stop 9 and 11; Fig.
1.21a-d). These occur in a few stratigraphic positions
indicating propagation of stronger tidal currents up
into the meandering and distributary channels during
certain phases of delta development while the shelf
area was not completely fi lled yet.
The Capella Fm contains tidally-infl uenced fl uvial
channels throughout its stratigraphy (Cuevas Gozalo
1985a,
b,
1989
; Cuevas Gozalo and de Boer
1991
) .
Typically, trough cross-stratifi ed sets in the deepest
part of the fl uvial channels that are tidally-infl uenced
have foresets showing mud drapes, bundling, reactiva-
tion surfaces not formed by dune overtaking and bipo-
lar (but unevenly distributed) current directions while
fl uvially generated cross-stratifi ed sets are clean,
coarse and unidirectional. Tidally-infl uenced point
bars contain large-scale avalanching foresets, under-
mined banks and bioturbated tops (de Boer
1998
) .
Additionally, Cuevas Gozalo (
1985a,
1989
, her page
80/81) and Cuevas Gozalo and de Boer (
1991
) defi ned
ebb-oriented, tidally-infl uenced fl uvial spill-over
lobes that formed in areas of fl uvial fl ow expansion
associated with fl uvial channel bifurcation on the
intertidal plain (Fig.
18.24
). They are single sedimen-
tary bedforms; the feeder channel shallows towards
the lobe fl at (Cuevas Gozalo
1985a,
1989
) .
Furthermore, in some cases an upstream transition
from a sand-dominated part of the point bar to sandy-
muddy part of the point bar is observed. Cuevas
Gozalo and de Boer (
1991
) suggest that upstream
fi nes deposited in the upper part of the point bar were
protected by fl ood dominated swales and/or inner
parts of the channel.
